Trench isolation is fast becoming the standard means of isolation in complex semiconductor devices, replacing the well established LOCOS (Localized Oxidation of Silicon) method of isolation. In a standard trench isolation process, trenches are formed in a semiconductor substrate between what are to become active areas that need to be isolated from one another. The trenches are filled with an insulating material, such as oxide, to provide electrical insulation. Active devices, including transistors and resistors, are then built and formed on and over the semiconductor substrate in appropriate active regions and in-between the isolation trenches.
One problem with standard trench isolation processes is the difficulty in forming borderless contacts over the trench regions. A borderless contact is a contact which overlies and exposes both the active and isolation regions of the semiconductor substrate, usually for the purpose of making contact to a diffusion region formed in the substrate. The problem of forming borderless contacts in combination with trench isolation involves the difficulty in etching contact openings in interlayer dielectrics while maintaining integrity of the trench isolation regions. For example, in order to make a contact to a diffusion region which abuts a trench isolation region, an opening must be formed in overlying interlayer dielectric layers to expose the diffusion region. In the case of a borderless contact, the contact opening will also overlie and expose a portion of the trench isolation region. Because the interlayer dielectric materials and the material used to form the trench isolation are generally very similar (both being primarily silicon dioxide), the trench isolation material is often etched during formation of the contact opening. The result is to recess the trench isolation material along the trench sidewalls. The recessed isolation is problematic upon subsequently depositing a conductive material into the contact opening. If the trench isolation material is recessed to the extent that it falls below the adjacent diffused region, a P-N junction will be exposed along the trench sidewall. Upon depositing a conductive material in the contact opening, the P-N junction will be electrically short circuited, rendering the device inoperable.
One solution to the problem of forming borderless contacts over trench isolation regions is to form a silicon nitride etch stop layer over the device prior to depositing the interlayer dielectric. In etching the contact opening, a first etch chemistry is used to etch through the interlayer dielectric, stopping on the silicon nitride etch stop layer. A subsequent etch chemistry is used to remove the silicon nitride etch stop layer within the contact opening to expose the diffused region. Because the etch stop layer also overlies the trench isolation region, the isolation material is protected from the interlayer dielectric etch and does not become recessed. However, problems with the use of such an etch stop layer are that 1) a chemistry selective to the etch stop must be developed and 2) eventually, the etch stop must also be removed to make contact to the active regions. Furthermore, the etch stop removal chemistry still has the potential to recess the trench fill oxide and expose the trench sidewall.